Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



there was mold on concrete cold cellar ceiling and the people who removed it also chiseled out some parts of the concrete. the ceiling is the floor of the front porch. could this now cause some structural damage. how do we fill in those holes left behind to get a smooth surface?
- Maria Leach
Sunday, October 16, 2016 at 4:00 PM

Kenneth, you could set a subfloor in the area. However, you will need to consult you local building code as to what size your footers will need to be to hold the weight of the wall. Your typical concrete is only poured 4" thick. This would not be strong enough to support the load of the wall without cracking your concrete below. But a footer would be a lot better for your application than filling the entire area with concrete.
- Lee-Technical Service
Thursday, October 13, 2016 at 10:42 AM

Garret, Top N Bond both can be applied from 1/2" down to a featheredge.The surface must be clean and free of any loose material, dust, debris, paints, sealers, and oils. The material also needs to be applied while the surface is damp.
- Lee-Technical Service
Wednesday, October 12, 2016 at 4:06 PM

We have a single carport 12ft wd x 18ft lg on a concrete slab that we want to enclose 8ft lg of it into a utility room. The back door of the house enters at the right front of the carport which will eventually serve as the entry to the utility room. Carport drops down 2ft at the back door. There is a partial back wall and side wall (with a row of brick at the bottom) on the outside and, of course, the wall to the house. So I will be basically be building part of an exterior wall and a 12ft wall across the inside of the carport. Question is, is it better to form up and fill in the whole area? That's a lot of concrete!! Or, how can I form a retaining wall for the two walls to seal off and then lay floor joists inside to set a marine plywood floor?
- Kenneth
Wednesday, October 12, 2016 at 1:02 PM

We are converting an old carport into a sunroom. The existing concrete slopes (understandable to shed water) unfortunately, my door has a 1/4" to 1/2" gap under the door. Most of the floor is level. I only have about a 5'x12' section to level. What is the best product to use for this situation? I need something I can feather as well.
- Garret
Monday, October 10, 2016 at 3:42 PM

I'm going to be installing stone veneer over a poured concrete wall. The wall is about 5 years old and never been painted or sealed. Can this product be used to prep the wall before installing the stone or do I need to use a wire mesh and scratch coat first?
- Ken Champ
Saturday, October 8, 2016 at 3:14 PM

David, the B1 Leveler is an interior only product. You will need to use Top'n Bond in multiple layers. Allow each layer to dry for at least 45 minutes and all the layers must be done within 24 hours.
- Lee-Technical Service
Wednesday, October 5, 2016 at 3:36 PM

Jeanette, with limited information, like the actual size and depth of the cracks, it is hard to make an exact recommendation for which product you would use. However, for movement cracks we would typically recommend the Non-Sag Polyurethane caulk. You would stuff the crack with a closed cell backer rod which can usually be for in the weather-stripping area. Then fill the crack with the Polyurethane. This material is very flexible and will allow for some movement from expansion and contraction with the change of the seasons. This is all providing that the crack is not larger than the limitations of the material.
- Lee-Technical Service
Wednesday, October 5, 2016 at 1:51 PM

I have a porch we are putting a sidder in the problem is on one side call it the left side facing the slidder is art zero and the right side is lower 2.5 inches.I have the liquid bonding agent and b levelor 6 bags.I have to bring the right side from 2.5 inche to zero in a 12 foot span can i use a concrete to fill in the realy low spots and the b levelor to get from 1 inch to zero. the issue i have is not enough b levelor?
- david corelli
Tuesday, October 4, 2016 at 7:41 PM

I have a pool deck that has subsided away from the pool coping as well as some fairly wide cracks. Can I use top n bond to build up deck to coping and fill and feather cracked areas?
- Jeanette
Saturday, October 1, 2016 at 12:51 AM



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